1. Field of the Invention
The present invention relates to the structure of the tooth profile of an internally meshing planetary gear structure suitable for a small sized reduction or speed up gear, and also relates to a flexible meshing type gear meshing structure, that is, the so-called wave gear structure.
2. Description of the Prior Art
There has been known a parallel type internally meshing planetary gear structure. It includes a first shaft, eccentric bodies rotated by the first shaft, plural external gears mounted around the eccentric bodies through bearings so as to be eccentrically rotatable, an internal gear internally meshing with the external gears through internal teeth constituted of external pins, and a second shaft connected to the external gears through internal pins for taking out only the rotational component of the external gears.
One known arrangement of this structure is shown in FIGS. 11 and 12. In this structure, the first shaft is taken as an input shaft, and the second shaft is taken as an output shaft. Further, the internal gear is fixed. Thus, the above structure is applied to the reduction gear.
Eccentric bodies 3a and 3b are fitted to an input shaft 1 with a specified phase difference (180.degree., in this structure). The eccentric bodies 3a and 3b (center: O.sub.2) are eccentric with the input shaft 1 (center: O.sub.1) by an eccentric amount e, respectively. Two external gears 5a and 5b are mounted around the eccentric bodies 3a and 3b through bearings 4a and 4b in a parallel manner (double row manner). Plural internal roller holes 6a and 6b are provided on the external gears 5a and 5b, respectively. An internal pin 7 and an internal roller 8 are inserted in each of the internal roller holes 6a and 6b.
The disposition of two external gears (double row) is mainly intended to increase the transmission capacity, to keep the strength, and to hold the rotational balance.
External teeth 9, each having a trochoid tooth profile (epitrochoid parallel curve tooth profile), are provided around the outer periphery of each of the external gears 5a and 5b. The external teeth 9 internally mesh with an internal gear 10 fixed to a casing 12.
The internal pins 7 passing through the external gears 5a and 5b are rigidly fixed to or inserted in a flange portion of the output shaft 2.
As the input shaft 1 is rotated by one time, the eccentric bodies 3a and 3b are rotated by one time. By one rotation of the eccentric bodies 3a and 3b, the external gears 5a and 5b are intended to be swayingly rotated around the input shaft 1; however, since the rotation is restricted by the internal gear 10, the external gears 5a and 5b are substantially only swayed while internally meshing with the internal gear 10.
For example, assuming that the teeth number of the external gears 5a and 5b is taken as n (n=21, in this figure) and the teeth number of the internal gear 10 is taken as n+1, the difference in the teeth number is 1. Accordingly, for each rotation of the input shaft 1, the external gears 5a and 5b are shifted (rotated) from the internal gear 10 fixed to the casing 12 by one tooth. This means that one rotation of the input shaft 1 is reduced to the rotation of -1/n of the external gears 5a and 5b.
In the rotation of the external gears 5a and 5b, the swaying component is absorbed by gaps among the internal roller holes 6a and 6b and internal pins 7 (internal rollers 8), and only the rotational component is transmitted to the output shaft 2 through the internal pins 7.
As a result, the reduction with a reduction ratio of -1/n is achieved.
The internally meshing planetary gear structure described above is applied to various types of reduction or speed up gears. In the above structure, the first and the second shafts are respectively taken as the input and output shafts, and the internal gear is fixed. However, the first shaft and the internal gear may be respectively taken as the input and output shafts, and the second shaft is fixed, to constitute the reduction gear. Further, in the above structures, the speed up gear may be constructed by replacing the input side by the output side.
In the internally meshing planetary gear structure of this type, the allowable load is almost determined depending on the magnitude of the surface pressure applied on the tooth surface. This has a limitation to the miniaturization of the device and to the enhancement of the load capacity. Thus, it has been required to reduce the surface pressure applied to the tooth surface.
To meet the above requirement, there has been proposed such a technique as disclosed in Japanese Patent Publication No. sho 63-4056. In this technique, an epitrochoid parallel curve (see FIG. 14) is used as the tooth profile of external teeth of an external gear, and a trochoid internally enveloping line (see FIG. 15) is used as the tooth profile of internal teeth 11 of an internal gear. As a result, the meshing points (contact points) between respective teeth of the external gear and the internal gear are increased to be two points. This makes it possible to reduce the surface pressure applied to the tooth surface.
More specifically, in this technique, as shown in FIG. 13, the tooth profile of the external teeth 9 of the external gears 5a and 5b is constituted of an epitrochoid parallel curve. On the other hand, the tooth profile of the internal teeth 11 of the internal gear 10 is constituted of a trochoid internally enveloping line composed of circular arc tooth profile portions P and P at both ends and a tooth profile portion Q at the intermediate portion (this portion is mated to the tooth profile of the external teeth which is constituted of the epitrochoid parallel curve).
In the gear structure using the above tooth profiles, at the meshing portion between the internal gear 10 and each of the external gears 5a and 5b, the contact points (meshing points) becomes two points in the area effective for the load transmission. Namely, there appears the contact point at the tooth profile portion Q other than the circular arc tooth profile portion P. Since these two contact points satisfy the condition of the tooth profile in terms of the gear mechanism, they act effectively to transmit the power.
Thus, in the internally meshing planetary gear structure of the above prior art, the improvement of the tooth profiles makes possible the contact at two points. Therefore, it is possible to reduce the surface pressure applied to the tooth surface, resulting in the miniaturization of the device and in the enhancement of the load capacity.
However, even using the above tooth profiles, there is a limitation to the enhancement of the load ability on the tooth surface yet. Thus, the higher load ability on the tooth surface has been required to further achieve the miniaturization and lightening of the reduction gear.
On the other hand, as a flexible meshing type gear meshing structure, there has been known such a technique as described in Japanese Patent Laid-open No. sho 63-130949. Hereinafter, this technique will be explained.
FIG. 16 is a sectional view showing the structure according to the prior art; and FIG. 17 is a sectional view taken along the line XVII--XVII of FIG. 16. In general, this structure is called as a wave gear structure.
An external spline 22A is provided on an input rotating shaft 21. It is connected to an internal spline 22B provided on an eccentric body 23 as a wave generator. An eccentric body bearing 24 is provided around the outer periphery of the eccentric body 23. An external gear 28 is provided around the outer periphery of the eccentric body bearing 24. The external gear 28 is constituted of a flange portion 29, annular portion 30 and an external teeth portion 31. The external teeth portion 31 is positioned around the outer periphery of an outer ring 27 of the eccentric body bearing 24.
In the above, the outer ring 27, annular portion 30 and the external teeth portion 31 are able to be flexibly deformed. External teeth 31A provided around the external teeth portion 31 are constituted of a trochoid tooth profile or the like. The external teeth 31A mesh with internal teeth 33A constituted of pins rotatably supported by the internal gear 32.
The external teeth 31A is smaller in the teeth number than the internal teeth (pins) of the internal gear 32 by two pieces. The configuration of the external teeth 31A is so constructed that two of the epitrochoid curves in which the radius ratio between the rolling circle and the base circle is an integer are superposed to be shifted in phase from each other, and the innermost one of the superposed curves is taken as the tooth profile curve. This configuration is disclosed in Japanese Patent Publication No. 1208548.
The rotation is transmitted from the input rotating shaft 21 to the wave generator (eccentric body 23). The eccentric body 23 deforms the external teeth portion 31 of the external gear 28 through the eccentric gear bearing 24. As the external teeth portion 31 is deformed by the projecting portions of the eccentric body 23, the external teeth 31A mesh with the internal teeth (pins) 33. Accordingly, the external teeth portion 31 is shifted in phase by the difference in the teeth number between the external teeth 31A and the pins 33A during one rotation of the eccentric body 23. By such shifting, the external gear 28 is rotated, which is transmitted to the output shaft 34. Specifically, in this example, the teeth number of the external teeth 31A is 100, and the teeth number of the internal teeth (pins) 33A is 102. Consequently, the difference in the teeth number is 2, so that the reduction ratio is (-2/100=-1/50).
Here, the internal gear 32 is fixed. However, as for the external gear 28 and the internal gear 32, if one is fixed, the other relatively becomes the output side. Also, if the output shaft is taken as the input side, the input rotating shaft becomes the output shaft for taking out the increased output.
In the prior art gear device as explained above, the internal teeth (pins) 33A constituting the circular arc tooth profile portion of the internal gear mesh with the external teeth 31A of the external gear 28 having the trochoid tooth profile or the like. Accordingly, the internal gear 32 is contacted with the external gear 28 at one contact point.
As a result, in the internal gear 32 constituted of the circular arc tooth profile having the same radius, the allowable load is almost determined depending on the restriction of the surface pressure applied to the tooth surface. This has a limitation to the miniaturization of the wave gear device and the enhancement of the load capacity.
In a planetary reduction gear using a circular arc tooth profile (external pins) as the internal gear tooth profile and a torochoid teeth profile as the external gear tooth profile (this is well known as the trademark registration "CYCLO" reduction gear according to the present applicant), as described above, the above problem (limitation in the load applied to the tooth surface) has been solved by the improvement of the tooth profile as shown in Japanese Patent Publication No. sho 63-4056.
Namely, as described above with reference to FIG. 13 showing the meshing state of the above tooth profiles, in this planetary reduction gear, the epitrochoid parallel curve (see FIG. 14) is used as the external gear tooth profile 9 and a trochoid internally enveloping line (see FIG. 15) is used as the internal gear tooth profile 11. This makes it possible to increase the meshing points between the external gear and the internal gear at two points, and hence to achieve the improvement of the load applied on the tooth surface.
Accordingly, it may be considered that the above problem is solved by applying the above tooth profiles to the above wave gear device.
However, in the above planetary reduction gear, the difference in the teeth number between the external gear and the internal gear is one. On the other hand, in the wave gear device, the difference in the teeth number must be an integer being two or more. Namely, in the wave gear device, it is preferred that the difference in the teeth number is the integer magnification of the number of the projecting portions of the eccentric body 23 as the wave generator. Since the number of the projecting portions of the eccentric body is two or more for balancing the load, so that the difference in the teeth number becomes an integer being 2 or more.
Therefore, the above tooth profiles cannot be directly applied to the wave gear device as they are.